Our work is directed towards understanding the basic mechanism of the actin-myosin interaction. Our approach has been to isolate the individual contractile proteins, and characterize their structural and functional properties by a combination of chemical, physical-chemical and immunological techniques. The basic problems remaining to be solved are: How is the energy of ATP hydrolysis converted into a mechanical force? How is the interaction between myosin and actin regulated? And how does the assembly of actin and myosininto filaments affect the basic actomyosin interaction? Our cntribution towards these questions will be to examine the properties of an individual cross-bridge, i.e. the head region of myosin, in as much detail as posible; specifically, we will study the role of the low molecular weight subunits in myosin; the effect of phosphorylation on the actomyosin system; the effect of calcium on the regulatory activity of myosin from vertebrate skeletal and smooth muscles; and the contribution of proteins other than myosin to the assembly and function of the thick filament. The composition of individual muscles will be analyzed by an immunocytochemical approach; and particularly homogeneous muscles selected for the preparation of contractile proteins for crystallization studies.